acpt: add blk-add-table.feature

Also reorganized acceptance tests a bit.
test: reorganize unitdata modules

No code changes.
test: incorporate declarative BaseBuilder
test: rewrite test for _Body.paragraphs
test: rewrite test for _Body.add_paragraph()
test: rewrite test for _Body.clear_content()
body: add _Body.tables
tbl: add Table.add_column()
tbl: add Table.add_row()
tbl: add _Body.add_table()
acpt: add tbl-item-access.feature
shr: add lazyproperty decorator
tbl: add Table.rows

Also migrate _tbl_bldr() methods to module level so they can be shared
by Describe_RowCollection.
tbl: add _RowCollection magic methods
acpt: add Row.cells access scenario
tbl: add _Row.cells
tbl: add _CellCollection magic methods
acpt: add Table.columns access scenario

Along the way, make collection scenarios parallel
tbl: add Table.columns
tbl: add _ColumnCollection magic methods

Along the way:

* refactored Describe_RowCollection for enhanced precision.
* replaced iter(list_comprehensions) in __iter__ methods with a
generator expression.